1. Field of the Invention
The invention relates to an apparatus for removing dust from gas, in particular from gas passed through a particulate material.
The terms "crude gas space" and "clean gas space" used below are first explained here. The crude gas space is understood as meaning a space which is adjacent to at least one filter and contains the gas to be filtered and to be freed from dust and from which the latter flows into the filter. A clean gas space is then understood as meaning a space which is adjacent to the or at least one filter and which the gas enters after flowing through the filter or the filters.
The apparatus has at least one container enclosing a crude gas space and at least one filter projecting into the crude gas space. The apparatus may have, for example, a material container and may be formed for passing gas through the crude gas space and through a particulate material present therein and for moving and treating the material, it being possible, for example, for the material to be fluidized with the gas and/or to be moved by a rotor which bounds the crude gas space at the bottom and has a disk. The means may serve, for example, for moving and treating a particulate material which is intended for the formation of a drug containing at least one pharmaceutical active ingredient, i.e. at least one active ingredient having a biological effect. The or at least one active ingredient may be contained, for example, in the particulate material introduced in the solid, dry or moist state into the material container. However, it is also possible to spray the particulate material in the material container with a liquid which may contain a dissolved or dispersed active ingredient. In addition to the material container or instead of it, the apparatus may have a downstream dust remover and/or another filtration device with a container which, when used, does not itself contain a particulate material but to which gas to be freed from dust is fed from another material container containing a particulate material. The apparatus may furthermore be formed as a spray dryer.
2. Description of the Prior Art
A fluidized-bed apparatus disclosed in U.S. Pat. No. 4,645,520 has a container which contains a fluidizing and crude gas space bounded at the top by an inner wall part. Filters projecting into the crude gas space are fastened to the inner wall part. Each filter has a casing with a cylindrical support member having gas passages, a filtration member arranged on the outside of said support member and supported by said support member, and bands serving for fastening said filtration member to the support member. The filtration member is formed by a flexible hose which has folds and comprises paper or a textile fabric. Each filter bounds a filter interior which is connected to a clean gas space present above the inner wall part by a passage passing through the inner wall part. A cleaning gas inlet which projects from the clean gas space through the passage into the filter interior, almost as far as the lower end of the filter, and has an inflatable bellows for closing the passage if required is present for each filter.
This known apparatus has the disadvantage that the filtration members consisting of paper or of a textile fabric have only low strength and a short service life and furthermore are not resistant to high temperatures. There is in particular a considerable danger that the filtration members will be damaged when the filter is blown out. The cleaning gas must therefore exert only a relative low pressure on the filtration members. However, a low cleaning gas pressure permits only port cleaning of the filtration members. In addition, the folds in the filtration members makes them more difficult to clean. The known apparatus furthermore has no devices for washing the filters inside the container. It would also be virtually impossible to wash the filters of the known apparatus clean inside the container by spraying with cleaning liquid under pressure, because this would even further shorten the service life of the filtration member. The filters must therefore each be removed from the container for washing, which is very time-consuming and may contaminate the environment and the operator.
If a bellows of the apparatus disclosed in U.S. Pat. No. 4,645,520 is to close a passage completely, it must be inflated and deformed to a very great extent. The bellows therefore probably closes the passage in practice only poorly, so that a considerable part of the cleaning gas flowing into the filter interior flows through the passage into the clean gas space during cleaning of a filter and does not contribute toward cleaning of the filter. The bellows are subjected to considerable stress during inflation and subsequent contraction and suffer considerable wear and must therefore probably be replaced after only a relatively short operating time. Furthermore, the bellows presumably consisting of natural and/or synthetic rubber are scarcely resistant to high temperatures and are brittle at low temperatures. In addition, the filtration members and bellows formed according to U.S. Pat. No. 4,645,520 are not sufficiently pressure-, heat- and fire-resistant to withstand any dust and/or solvent explosion which may take place in the crude gas space, so that they will be destroyed by such an explosion and will allow the explosion to spread through the filter.
Other known fluidized-bed apparatuses and other apparatuses for removing dust from gas likewise have filters with filtration members which consist of flexible hoses comprising paper or a textile material and generally have axial or circular or helical folds. These filters have disadvantages substantially similar to those of the filters described above and disclosed in U.S. Pat. No. 4,645,520.